1. Field of the Invention
This invention relates to vented casting molds and to processes of making the same, particularly such molds useful for casting in an evacuated atmosphere by a process such as disclosed in U.S. Pat. Nos. 3,863,706 and 3,900,064, especially when very thin walled portions are to be cast.
2. Description of the Related Art
The casting cavities of investment molds need permeability for the escape of gases present in those cavities to insure complete fill-out of the cavities during filling. This is also true when the mold is being used in the counter-gravity, applied vacuum filling process disclosed in the aforesaid patents. For making such molds, a ceramic composition can be used which in its final hardened state has a permeability of the order of 14 cc nitrogen per square inch (6.5 cm.sup.2) of mold area per minute at a nitrogen pressure of 0.5 p.s.i. This provides adequate venting to insure filling of cavities of which the shortest transverse cross-section dimension is about 0.04 inch (1.0 mm) or more and for gas removal during casting as long as the area of this cross-section is less than a few square inches. However, it has been difficult or impossible to achieve complete fill-out of very thin portions having one transverse cross-section dimension less than 0.04 inches, especially those of greater area than three square inches (19.5 cm.sup.2).
Attempts to solve this problem by a mold-making composition which will provide molds of greater permeability have not proven satisfactory. The molds formed therewith have been weaker, prone to failure in casting, and the mold cavity surfaces are too porous, resulting in rough cast surfaces. Vent slots ground in, or molded through, the mold have a smallest transverse cross-section dimension so large that the material being cast leaks out unacceptably, and such grinding or molding are difficult and expensive. Molding thinner slots about a pattern in the "lost wax" process of forming the casting cavity is not feasible, because the extremely thin lost wax pattern for the slit would be too fragile to withstand embedding in the mold-forming composition and the outer end of the pattern would be embedded in that composition, plugging the slots.